In producing semiconductor on insulator structures, it is difficult to achieve direct bonding of certain semiconductor materials to a layer (for example an oxide layer). This difficulty is observed in particular when, in order to produce a silicon-germanium on insulator (SGOI) structure, a layer of silicon-germanium (SiGe) is to be bonded directly to a layer of oxide on the surface of a support substrate. This difficulty is all the more pronounced when the concentration of germanium in the SiGe layer is high.
In order to carry out direct bonding of a SiGe layer to a support substrate, methods have been proposed for treating the surfaces to be bonded prior to bonding. For example, in “SiGe-free strained silicon in insulator by wafer bonding and layer transfer”, Applied Physics Letters, vol 82 number 24, Jun. 16, 2003, T. A. Langdo et al. disclose bonding a layer of strained silicon on a layer of thermal oxide. Additionally, a method known as “hydrophobic bonding” may be employed, in which the surfaces to be bonded are prepared prior to bonding in order to render them hydrophobic (for example by immersing them in an HF chemical cleaning bath). However, this preparation method tends to cause severe particulate contamination of the surfaces, which can result in generating defects during bonding. A hydrophilic bonding method has also been proposed in which the surface of the SiGe layer to be bonded is treated to render it hydrophilic prior to bringing it into contact either with a hydrophilic silicon surface or with a surface of silicon oxide SiO2.
One of the best known treatments for rendering a layer hydrophilic consists in immersing the layer in a chemical SC1 type solution (NH4OH/H2O2/H2O). However, this solution etches the SiGe layer and tends to increase its roughness, resulting in a bond of unsatisfactory quality. Moreover, the etching rate varies exponentially as a function of the concentration of germanium in a SiGe layer. For this reason, although controlled etching in a SiGe layer having a low concentration of germanium (e.g. 20%) is possible (and thus bonding may be envisaged), controlled etching in a SiGe layer having a high concentration of germanium (e.g. 50%) is difficult. Finally, while hydrophilic bonding of a SiGe layer having a low concentration of germanium (for example 20%) may be carried out in a relatively satisfactory manner, hydrophilic bonding of a SiGe layer having a high concentration of germanium (for example 50%) cannot be achieved.
Known surface treatment methods for directly bonding an SiGe layer to a support substrate of silicon or SiO2 can result in bonding quality that is generally insufficient to meet certain durability specifications in certain fields of application. Good quality bonding is even more difficult to achieve when the germanium concentration in the SiGe layer to be transferred is high. Moreover, the above-mentioned disadvantages of direct bonding of a SiGe layer may also be encountered in the context of direct bonding of layers produced in other materials.
Indirect bonding can also be employed to form a sufficient bond. The term “indirect bonding” means bonding employing a bonding layer interposed between two layers of materials which are to be bonded together. The use of a bonding layer is of particular advantage when it is difficult to achieve direct bonding of certain semiconductor materials to a layer. Such bonding layers can comprise, for example, a layer of oxide or a layer of silicon. When the bonding layer is brought into contact with a surface of the support substrate the properties of the bonding layer permit good quality bonding to be achieved.
In order to carry out indirect bonding, advantage may be taken of the good bonding properties of (1) a layer of silicon on a layer of the oxide SiO2 on the surface of a support substrate, (2) a layer of oxide on a layer of the oxide SiO2 on the surface of a support substrate, or (3) a silicon support substrate.
When performing indirect bonding of a layer of SiGe to a support substrate; the surface of which may, for example, be formed from silicon or SiO2, a layer of oxide may be created directly on a layer of SiGe by oxidizing the SiGe layer. However, during oxidation of the SiGe layer, the germanium present in the layer is pushed into regions thereof which are distant from the oxide layer being formed. Thus, germanium is observed to be segregated at the SiGe/oxide interface. Also, a zone is formed opposite the oxide in which the overall concentration of germanium increases. Thus, the concentration of germanium is no longer uniform through the SiGe layer.
Although this germanium segregation phenomenon may be minimized, particularly by carrying out the oxidation at low temperatures, the oxide formed is thermally unstable. Thus, this technique cannot be used when forming a good quality SGOI structure.
Alternatively, instead of oxidizing a layer of SiGe to obtain a bonding layer, a further known method of indirect bonding includes directly depositing a layer of oxide onto a layer of SiGe. The deposited oxide layer must then be polished prior to bonding to a silicon support substrate. However, when employing this technique, the oxide layer deposited on the SiGe layer then constitutes the buried oxide layer of the SGOI structure obtained, which can be disadvantageous. The density of that oxide layer and its electrical performance may prove to be insufficient for producing an oxide/semiconductor interface that satisfies the durability/force specifications in the semiconductor industry, particularly compared with the density and performance of a thermal oxide. Moreover, the use of a deposited oxide can also result in poor homogeneity of the thickness of the buried oxide layer in the SGOI structure, which can also be undesirable. Further, depositing the oxide layer may also cause contamination resulting in electrical defects.
Another technique for indirectly bonding a layer includes epitaxially growing a layer of strained silicon on the surface of a layer of SiGe. That silicon layer is subsequently completely oxidized to form a layer of silicon oxide (SiO2), which permits oxide/oxide bonding with an oxidized silicon support substrate. Thus, total oxidation of the silicon layer epitaxially grown on the SiGe layer effectively causes the formation of an oxide layer on the surface of the structure to be bonded.
However, stopping the oxidation front of that epitaxially grown silicon layer exactly at the interface between the epitaxially grown silicon layer and the SiGe layer is difficult to attain. The oxidation front is thus generally stopped after the Si/SiGe interface. This is termed “overoxidation.” In such a case, SiGe is oxidized and the composite oxide SiyGetO2 is then formed, where y and t represent the respective concentrations of silicon and germanium in the composite oxide. Even more disadvantageously, the oxide/SiGe interface can then be susceptible of trapping charge carriers. While such a technique allows good oxide/oxide bonding, the effects of difficulty of controlling the oxidation front, coupled with a poor quality oxide/SiGe interface in the case of overoxidation, are not desirable.
Thus, there still remains a need in the art for a more effective technique for bonding a layer such as a Silicon-Germanium layer to a substrate. This need is now met by the present invention.